Characterization of the Metal-Binding Site in Aminopeptidase B

Abstract

A recombinant rat aminopeptidase-B (Ap-B) was expressed as a glutathione S-transferase (GST) fusion protein in Escherichia coli BL21 harboring a plasmid pGEX-Ap-B and was purified by glutathione-Sepharose 4B and Q-Sepharose columns. The metal-substituted derivatives of Ap-B, Co(II)- and Cu(II)-Ap-B contain almost 1 mole of cobalt(II) and copper(II) ions per enzyme molecule, respectively. The specific activity of Co(II)-Ap-B is very similar to that of recombinant Ap-B but that of Cu(II)-Ap-B is very low. The dissociation constants of the zinc ions of recombinant Ap-B and of the cobalt ions of Co(II)-Ap-B calculated from the relationships between the free metal ions and the residual enzyme activities are 3.7(±1.0)×10⁻¹³ and 4.7(±1.0)×10⁻¹² M, respectively. The EPR parameters (g_⊥, g_// and A_//) of Cu(II)-Ap-B were 2.06, 2.27, and 156×10⁻⁴ cm⁻¹. The A_// value and the g_// of Cu(II)-Ap-B are very similar to those of Cu(II)-thermolysin or Cu(II)-dipeptidyl peptidase III, in which the coordination geometry is a distorted tetrahedral.